In radiography it is important to have excellent image quality for the radiologist in order to make an accurate evaluation of a patient's condition. Important image quality aspects are image resolution and image signal-to-noise ratio (SNR).
For computed radiography (CR) SNR depends on a number of factors.
The number of X-ray quanta absorbed by the storage phosphor screen is important. SNR will be proportional to the square-root of the number of absorbed quanta.
The so-called fluorescence noise, however, is of primary importance as well. This noise contribution originates from the fact that the number of photostimulated light (PSL) quanta detected for an absorbed X-ray quantum is small. Since much of the PSL is lost in the detection process in CR, fluorescence noise has an important contribution to the SNR. Hence, it is important that the number of photons detected per absorbed X-ray quantum is as high as possible. This situation is most critical in mammography, where X-ray quanta are used with low energy. Softer X-rays will give rise to less PSL centres and, therefore, to less PSL photons per absorbed X-ray quantum than harder X-rays.
In CR, a large number of PSL centres is created by an absorbed X-ray quantum. However, not all PSL centres are stimulated in the read-out process, because of the limited time available for pixel stimulation and because of the limited laser power available.
Typically, only about 30% of the PSL centres is stimulated to give rise to a PSL photon. Since these photons are emitted and scattered in all directions, only 50% of the PSL photons escape from the storage phosphor screen at the detector side. Only a fraction of the PSL photons emitted at the top side of the storage phosphor screen is guided to the detector, which has a limited quantum efficiency itself. For that reason, the number of PSL photons detected per absorbed X-ray quantum is of the order of 1 to 5 and the fluorescence noise contribution is important in CR systems.
In addition, it is well-known that fine detail visualisation, i.e. high-resolution high-contrast images are required for many X-ray medical imaging systems and, more particularly, in mammography. In phosphor screens, light scattering by the phosphor particles and their grain boundaries results in loss of spatial resolution and contrast in the image.
The number of PSL centres that is stimulated in the read-out process can be increased by reflecting the stimulating light at the bottom of the phosphor layer, i.e. by having a reflecting substrate. In this case the fraction of PSL centres that is stimulated will be higher than 30%. A reflecting substrate will also reflect the PSL photons, thereby increasing the number that leaves the screen at the top side to a fraction higher than 50%. The combination of these effects may increase the number of PSL centres detected per absorbed X-ray quantum to a significant extent, thereby strongly improving the image SNR. However, having a reflecting substrate causes increased scattering in a powder screen as well. The stimulating light spot is broadened when it is reflected at the screen substrate and spatial resolution is diminished. In powder CR screens, therefore, a reflective substrate is seldom used as such. It may be used in combination with an anti-halation dye on top of it. The anti-halation dye absorbs the stimulation light, thereby preventing its reflection and maintaining high resolution. As a consequence of having the anti-halation dye on top of the reflective substrate, however, the sensitivity of the CR plate is not remarkably enhanced.